Abstract Voltage-dependent Na+ (Nav) channels typify the most distinctive feature of most neurons and other excitable, their capacity to generate action potentials (AP). In several classic cases, a primary role of Nav channels is to ensure failsafe reliability of the AP, whether in long distance AP propagation in an axon or in contributing to reproducibility of APs in skeletal or cardiac muscle. However, there is a growing realization that Nav channels in many neurons and neuroendocrine cells play central roles in the regulation of cell firing behavior, contributing to accommodation during repetitive firing with associated changes in AP height or duration. These effects on firing arise, in part, from use-dependent changes in Nav current availability defined by the specific inactivation properties of different Nav channels. Multiple kinds of fast inactivation have been described for Nav channels. One is classic fast inactivation that is intrinsic to the Nav pore-forming ? subunit. Another involves pore block by N-terminal segments of specific intracellular fibroblast growth factor homologous factors (iFGFs). When two forms of fast inactivation are present at the same time, they act in a competitive fashon. The differences in recovery from inactivation of the two forms then critically define Nav current availability. The extent to which iFGFs regulate properties of different Nav channels is only beginning to be understood. Some challenges are that there are multiple Nav variants in many cells and multiple FGFs, some inactivating, some noninactivating, that can compete for association with Navs. In this project, we will utilize a relatively simple cell, chromaffin cells (CCs) of the adrenal medulla, which have the excitability properties of neurons, but offer advantages for teasing apart the role of iFGFs. Using methods of electrophysiology coupled with genetic manipulations that delete specific subunits from mice, this project will define properties of inactivation of Nav current in CCs, tease apart the dual-pathway fast inactivation behavior observed in such cells, determine the identity of specific subtypes of Nav currents found in CCs, and assess the role of iFGFs in producing the unusual inactivation behavior. This project is expected to provide new insight into the properties of inactivation mediated by FGF's, the distinctions between normal fast inactivation and iFGF-mediated inactivation when both are present, potential specificity in the roles of different iFGFs and different Nav isoforms in regards to native cells, and the impact of cytosolic iFGF-mediated inactivation on cell excitability. As potential disease causing mutations in iFGF's or their associated Nav channels become revealed, the results of this project will be important to assessing the impact and potential future therapeutic strategies.